Originally Posted by kefrko

About 3 weeks ago I bought a new to me 2000 F350 crew cab with a long bed. So far I love the truck and have made a few changes to it. I bought a billet thermostat housing because the stock one was corroded and began leaking. Replaced the t-stat with a 203 degree t-stat. New MBPR 4" turbo back exhaust and did the Gooch's coolant flush with fleetrite ELC. Now that I got some of the important stuff out of the way I wanted to tackle some less important things. I noticed the vents would occasionally only blow out of the defrost no matter what the switch was set to. After much research on this site and similar sites I decided to trouble shoot it. I turned the key to the on position and the vacuum pump came on which I figured is a good start. After maybe 20 sec the pump shut off. I listened for any leaks but couldn't hear anything. I followed the lines but didn't see anything that looked too bad. Then I popped the glove box out of the way and looked for loose vacuum tubes. everything looked fine so I turned on the fan and sure enough the air was coming out of the correct vents. I went through the different vents a couple of times and it worked perfectly. I know I didnt fix the problem yet and I only happened to catch it on a good day. after a couple of cycles of changing the vents I did notice the vacuum pump come back on. My question (I know its been a long time coming) is, does the vacuum cycle on and off to maintain the pressure because changing the vents will use the vacuum and release it or is that a closed system and once it is under vacuum it should stay like that? I am guessing everytime you take it up to the defrost it has no vacuum and would lose the pressure. I havent tried running the truck and doing the same procedure yet to test the A/C That will be next. If it is a leak in the hoses (normally by the hubs) would it be best to start furthest away and test the hubs with a handheld vacuum pump to see if they engage and then work my way back to the reservoir?